ext4: use memweight()

Convert ext4_count_free() to use memweight() instead of table lookup
based counting clear bits implementation.  This change only affects the
code segments enabled by EXT4FS_DEBUG.

Note that this memweight() call can't be replaced with a single
bitmap_weight() call, although the pointer to the memory area is aligned
to long-word boundary.  Because the size of the memory area may not be a
multiple of BITS_PER_LONG, then it returns wrong value on big-endian
architecture.

This also includes the following change.

- Remove unnecessary map == NULL check in ext4_count_free() which
  always takes non-null pointer as the memory area.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext4/bitmap.c

@@ -11,16 +11,9 @@
 #include <linux/jbd2.h>
 #include "ext4.h"
 
-static const int nibblemap[] = {4, 3, 3, 2, 3, 2, 2, 1, 3, 2, 2, 1, 2, 1, 1, 0};
-
 unsigned int ext4_count_free(char *bitmap, unsigned int numchars)
 {
-	unsigned int i, sum = 0;
-
-	for (i = 0; i < numchars; i++)
-		sum += nibblemap[bitmap[i] & 0xf] +
-			nibblemap[(bitmap[i] >> 4) & 0xf];
-	return sum;
+	return numchars * BITS_PER_BYTE - memweight(bitmap, numchars);
 }
 
 int ext4_inode_bitmap_csum_verify(struct super_block *sb, ext4_group_t group,